Catalytic Decarboxylation of Petroleum Acids from High Acid Crude

The charge distributions, the chemical bond orders, and the energy barriers of petroleum acids with different structures were calculated using VAMP, ...
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Energy & Fuels 2008, 22, 1923–1929

1923

Catalytic Decarboxylation of Petroleum Acids from High Acid Crude Oils over Solid Acid Catalysts Xiaoqin Fu,*,† Zhenyu Dai,‡ Songbai Tian,‡ Jun Long,‡ Suandi Hou,‡ and Xieqing Wang‡ EnVironmental Monitoring Center of Ningbo, Ningbo, China, and Research Institute of Petroleum Processing, Beijing, China ReceiVed NoVember 2, 2007. ReVised Manuscript ReceiVed March 8, 2008

The presence of petroleum acids in crude oils is a key problem in refineries due to corrosion to industrial units. To economically and effectively remove the petroleum acids from high acid crude oils is an urgent project. A theoretical and experimental study to develop a catalytic decarboxylation process to remove petroleum acids from high acid crude oils was conducted. By using the molecular simulation technology, a solid acid catalyst was constructed for removing petroleum acids. The experimental results indicated that the acid removal rate of a high acid crude oil with a total acid number (TAN) of 12.52 over the solid acid catalyst of MLC500 was up to 97% at a volume space velocity of 8 h-1, ratio of catalyst to oil of 7.5, and temperature of 460 °C in the fixed fluid bed reactor.

Introduction Processing high acid crude oils often causes severe equipment corrosion in refineries. Petroleum acids in crude oils found in many countries and areas, such as China, India, Russia, the United States, West Africa, and North Europe, are considered to be the major source of the corrosion.1–3 The acid concentration in the crude oil, technologically termed total acid number (TAN), is defined as the amount of KOH in milligrams required to neutralize the acidity of one gram of oil. ASTM D974 and ASTM D664 are two methods for measuring TAN, especially the latter one which is more commonly used. Generally, the crude oils with a TAN higher than 0.5 are regarded as high acid crude. The petroleum acids mainly consist of monocarboxylic acids, which include aliphatic acids, naphthenic acids, and aromatic acids. Of those, naphthenic acids with five or six member rings account for approximately 85%.4–7 Table 1 lists some representative naphthenic acids. Removing petroleum acids from crude oils is one of the important challenges in processing high acid crude. Two industrial methods have been used. One is blending a high TAN crude oil with a low TAN one so that the TAN of the blend is no higher than 0.5. The other is neutralizing the acids with the solution of sodium hydroxide. The former limits the amount of acidic crude oils that can be processed. The latter is easy to form emulsions which are very difficult to treat in sequential * Corresponding author. † Environmental Monitoring Center of Ningbo. ‡ Research Institute of Petroleum Processing. (1) Ying, Xu. Corrosion Protection Pet. Ind. 2003, 20, 1–4. (2) Chunshu, L. I. Tot. Corrosion Control 2004, 18, 6–10. (3) Haynes, D. Hydrocarbon Asia 2002, 9, 34–39. (4) Dzidic, I.; Somerville, A. C.; Raia, J. C.; Hart, H. V. Anal. Chem. 1988, 60, 1318–1323. (5) Rudzinski, W. E.; Oehlers, L.; Zhang, Y. Energy Fuels 2002, 16, 1178–1185. (6) Laredo, G. C.; Lopez, C. R.; Alvarez, R. E.; Cano, J. L. Fuel 2004, 83, 1689–1695. (7) Fu, X.; Tian, S.; Hou, S.; Wang, X. Chem. Eng. Oil Gas 2007, 36, 507–510.

processing. With the increase of high acid crude in the market and its low price new technologies are required to further refine high TAN crude. A lot of methods, both patents and publication, have been proposed, but most of them still stop at the laboratory stage.8–11 Accordingly, the objective of this study is to develop a catalyst and process for catalytic decarboxylation of high acid crude. By means of molecular simulation technology,12 some of the molecular properties of naphthenic acids were calculated, and then the possible mechanisms of catalytic decarboxylation over solid acid catalyst were explained, finally the catalytic decarboxylation of petroleum acids present in a high acid crude was experimentally tested in this paper. Experimental Section Software Packages for Molecular Simulation. Cerius2 4.8, InsightII 4.0.0, and Material Studio 3.1 (Accelrys Inc., San Diego, CA). Molecular Simulation Methodology. The molecular mechanics and dynamics were determined through the universal field to optimize the structures of naphthenic acidic model compounds. The time step was preset as 1 × 10-12 s. For each model compound, one million steps were conducted during the simulation process to obtain a low energy structural image. The charge distributions, the chemical bond orders, and the energy barriers of petroleum acids with different structures were calculated using VAMP, a computing program of quantum mechanics based on the molecular orbital theory. Model Naphthenic Acids. Fourteen model naphthenic acids (seen in Table 1) were selected for molecular simulation. Materials. Material A with a TAN of 5.12 and material B with a TAN of 24.38 were prepared by dissolving the petroleum acids (8) Zhu, X.; Tian, S. Corrosion Protection Pet. Ind. 2005, 22, 7–10. (9) Fu, X.; Tian, S.; Hou, S.; Wang, X. Chem. Ind. Eng. Progr. 2005, 24, 968–970. (10) Shen, H.; Wang, Y.; Li, R. Pet. Sci. 2004, 1, 78–81. (11) Zhang, A.; Ma, Q.; Wang, K. Appl. Catal. A:Gen. 2006, 303, 103– 109. (12) Frenkel, D.; Smit, B. Understanding of molecular simulation; 2002, pp 1-7.

10.1021/ef7006547 CCC: $40.75  2008 American Chemical Society Published on Web 04/30/2008

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Table 1. Structures of Some Representative Naphthenic Acids

with a TAN of 200 purified from the diesel oil in the jet fuel. Material C with a TAN of 24.54 was prepared in the same method, only the petroleum acids with a TAN of 120 were purified from the fourth side-line of the vacuum tower. Material D is a high acid crude with a TAN of 12.52. Equipment and Experimental Method. A scheme of the fixed bed reaction system was depicted in Figure 1. The decarboxylation catalyst was first loaded in the constant temperature section of the fixed bed reactor. The reactor filled with nitrogen was heated to temperature of 150 to keep for 2 h and then further heated to the reaction temperature. The material oil mentioned above was pumped

Figure 1. Scheme of the fixed bed reaction system.

into the reactor. Finally, the condensed oil fraction was collected from the reactor and analyzed to determine the total acid number (TAN). A scheme of the fixed fluid bed reaction system was illustrated in Figure 2. The catalyst filled in the reactor was heated to the desired reaction temperature. The material oil was then pumped into the reactor, while the atomizing steam was flowed into the reactor at the same time. The condensed oil fraction was collected from the reactor and analyzed to determine the total acid number. Acid Removal Rate. The efficacy of petroleum acids removed by the reaction was defined as the acid removal rate R:

Catalytic Decarboxylation of Petroleum Acids R)

(x1 - x2) × 100 % x1

where R is acid removal rate (%); x1 is the TAN of the feed; and x2 represents the TAN of the product.

Results and Discussion It is well-known that solid catalysts possess the function of catalytic cracking reactions for petroleum hydrocarbons.13 If they possess the function of catalytic decarboxylation for petroleum acids as well, the goal of this study will be fulfilled because high acid crude oils belong to heavy oils. Benzoic acids can be catalytic decarboxylated over zeolite catalysts.14 Therefore, solid acid catalysts were selected for studying catalytic decarboxylation. The research results of nonhomogeneous catalysis materials indicate that solid acid catalysts include both Bro¨nsted acid and Lewis acid.13 In order to study the characteristics of catalytic decarboxylation reactions of petroleum acids over solid acid catalysts, some molecular properties of petroleum acids, such as bond strength and charge distribution, need to be investigated in advance. According to the molecular orbital theory,15 the bond strength can be used to express the bond order. Bond Orders and Charge Distributions of Model Naphthenic Acids. The bond orders and the charge distributions of the fourteen naphthenic acids were calculated with the molecular simulation technology, and three typical examples

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were given in Table 2 for reference. The simulation results showed that in naphthenic acid bond order of C-O was higher than that of C-C, which means that the C-O bond was more stable and difficult to be broken and the bond order of C-C connected with a carbon atom in the carboxyl was the lowest, which also means that this C-C bond was the easiest to be broken compared with other C-C bonds. Therefore, the carboxyl of petroleum acid was easy to be removed as carbon dioxide. The simulation results also indicated that the carbon atom of carboxyl had a positive charge of about 0.39 while its oxygen atom had a negative charge of around 0.41, and thus the negative charges were concentrated on carboxyls. Two above results were found universal in the fourteen naphthenic acids, which suggested that different naphthenic acids possess similar chemical reaction characteristics. Possible Mechanism of Catylytic Decarboxylation by Bro¨nsted Acid. Bro¨nsted acid is one of two acid centers of solid acid catalysts, and here, proton stands for Bro¨nsted acid. As we know from above molecular simulation results, in naphthenic acids, negative charges were concentrated on carboxyls of which oxygen atoms had more negative charges. Therefore, with the existence of Bro¨nsted acids, protons (i.e., Bro¨nsted acidic centers) took the priority to be combined with oxygen atoms of the carbonyls. Again, the simulation results showed that the distance between the carbon atoms in the carbonyl and methyl was elongated according to the quantum mechanical calculations. Taking the model compound m01 as

Table 2. Bond Orders and the Charge Distributions of Three Model Naphthenic Acids

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Figure 2. Scheme of the fixed fluid bed reaction system.

Figure 3. Change of C-C bond order and length for model compound m01 before and after being combined with a proton.

an example, the bond length of its C-C varied from 0.1517 nm before the combination between the proton and the oxygen atom to 0.2299 nm after the combination, as shown in Figure 3. This showed that the cleavage of C-C bond occurred, as evidenced by the decrease of bond order from 0.928 (before the combination) to 0 (after the combination). Figure 4 demonstrated the possible pathway for catalytic decarboxylation of petroleum acids by Bro¨nsted acid. The carboxyls in petroleum acids were inclined to combine with protons. As a result, the distance between the carbon atoms in the carbonyl and methyl groups became longer and longer, which eventually leading to the detachment of CO2 groups from petroleum acids. Possible Mechanism of Catalytic Decarboxylation by Lewis Acid. Lewis acid is another acid center of solid acid catalysts. Figure 5 showed the well-known Lewis acidic centers that were constructed according to the compositions and structures of solid acid catalysts.16 (13) Chen, Y. Fluid Catalytic Cracking Technology and Engineering, second ed.; China, 2005; pp 126-221. (14) Takemura, A.; Nakamura, H.; Taguchi, K. U. Ind. Eng. Chem. Prod. Res. DeV. 1985, 22, 215–312. (15) Zhou, G.; Duan, L. Principles of Quantum Mechanics; 2002; p 84.

On the basis of the computations mentioned above, the negative charges were concentrated on the carboxyls of petroleum acids, and the Lewis acidic centers had low-energy empty orbits. Therefore, Lewis acids tended to be combined with petroleum acids. Taking model compound m01 as an example, the energy dropped to 103.37 kJ/mol after the combination between m01 and Lewis acids. It demonstrated that petroleum acids were ready to be adsorbed onto Lewis acidic centers through the interactions between oxygen atoms and the empty orbits in Lewis acids. Once this happened, the hydrogen atoms in the hydroxyl tended to get closer to the R-carbon atoms in the carboxyls and eventually were separated from the carboxyls, as shown in Figure 6. The possible catalytic decarboxylation pathway for petroleum acids by Lewis acids was illustrated in Figure 7. Because the oxygen atoms in the carboxyls of petroleum acids had lone-pair electrons as well as abundant negative charges, and the aluminum atoms in the molecular sieve skeleton had empty orbits, the petroleum acids were easy to be absorbed onto the aluminum surfaces. With the aid of the (16) Xu, R.; Pang, W. Zeolites Porous Materials Chemistry; China, 2004; pp 70-77.

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Figure 4. Mechanism of catalytic decarboxylation by Bro¨nsted acid.

Figure 7. Mechanism of catalytic decarboxylation by Lewis acids. Figure 5. Structure of Lewis acid.

Figure 6. Structures of m01 before and after the adsorption onto Lewis acidic centers.

hydrogen atoms in the hydroxyls, the carboxyls were then removed from the petroleum acids. Mechanism of Thermal Decarboxylation of Petroleum Acids. For clearly explaining the process of catalytic decarboxylation of naphtheinc acids on solid acid catalysts, it is necessary to introduce the thermal decarboxyltion of naphthenic acids. The thermal decarboxylation of petroleum acids is generally considered to be a two-stage process, as illustrated in Figure 8.17 Petroleum acids first lose protons to produce

Figure 8. Mechanism of thermal decarboxylation of petroleum acids.

negatively charged carboxyl ions which then undergo heterogeneous cracking reactions to release carbon dioxides and

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Table 3. Energy Barriers of Thermal Decarboxylation and Catalytic Decarboxylaton of Model Compounds of Naphthenic Acids model compounds of naphthenic acid

energy barriers of thermal decarboxylation (kJ · mol-1)

energy barriers of catalytic decarboxylaton by Bro¨nsted acid (kJ · mol-1)

energy barriers of catalytic decarboxylaton by Lewis acid (kJ · mol-1)

M01 M02 M03 M04 M05 M06 M07 M08 M09 M10 M11 M12 M13 M14

339.29 384.88 348.26 316.01 336.90 317.28 348.91 385.47 334.47 378.37 338.01 316.44 341.35 376.08

276.95 235.06 239.63 254.84 271.11 270.35 272.43 251.74 267.27 271.67 233.12 242.67 275.44 233.79

116.87 181.67 163.06 221.66 124.27 108.65 158.65 179.62 114.78 116.54 118.45 184.14 122.06 183.91

negatively charged alkane ions. Finally, the negative alkane ions combine with protons to form alkanes. Energy Barriers of Thermal Decarboxylation and Catalytic Decarboxylation of Naphthenic Acids. A chemical reaction always goes through an activation process in which the reactants must gain sufficient energies to reach their transient formations and then span the energy barriers to be turned into the final products. The energy barrier is defined as the energy difference between the transient formations and the reactants. On the basis of the chemical reactions of thermal decarboxylation and catalytic decarboxylation by Bro¨nsted acid and Lewis acid, the energies of reactants and their transient formations were calculated respectively using VAMP of quantum-mechanicals. Thus, the energy barriers of thermal decarboxylation and catalytic decarboxylaton by Bro¨nsted acid and Lewis acid were obtained. The energy barriers of 14 model compounds of naphthenic acids were presented in Table 3. As shown in Table 3, the energy barriers of catalytic decarboxylation for naphthenic acids by Lewis acid were the lowest with a range from 100 to 200 kJ · mol-1. Those by Bro¨nsted acid were the midst ranged from 230 to 280 kJ · mol-1. The highest were those by thermal decaboxylation with a range from 310 to 380 kJ · mol-1. The decaboxylation of naphthenic acids were much easier to occur over acid catalysts in comparison with those by thermal decarboxylation. The catalytic decarboxylation efficiency by Lewis acid was better than that by Bro¨nsted acid. Acid Removal Rate of High Acid Crude Oils over Solid Acid Catalysts. On the basis of the computation results, experiments for investigating catalytic decarboxylation were conducted in the fixed bed reactor. The scheme of the experimental unit was illustrated in Figure 1. The reaction temperature and the volume space velocity of the experiments were 350 °C and 3 h-1, respectively. The acid removal rates of material A over the activated alumina (γ-Al203) (representing the Lewis acid18) and over the quartz sands (standing for a heat carrier) were investigated. The relationship between the acid removal rate and the running time was also observed. These experimental data were shown in Figure 9. The acid removal rate of thermal decarboxylation over quartz sands was only 40% while that of catalytic decarboxylation over the activated alumina (γ-Al203) was as high as 84%. In both cases, the acid removal rate did not change with the running (17) Xia, Q.-t. J. Qian Nan Normal College of Nationalities 2004, 6, 15–19. (18) Zen, K.; Wang, G.; Bi, Y.; Li, R. Fundamentals of Catalysis and Applications; China, 2005; pp 111-113.

time. Theses results showed that the decarboxylation was easier to occur when catalyzed by acid catalysts in comparison with that by thermal decarboxylation. However, the reason why the acid removal rate could not reach 100% may be that the petroleum acids could not arrive at the active centers of catalysts for decarboxylation. Catalysts in a fixed fluidized bed have larger specific surface areas and pore volumes than those in a fixed bed. Therefore, much more petroleum acid can arrive at the active centers of catalysts in a fixed fluidized bed than in a fixed bed. Thus, experiments were also conducted in fixed fluidized bed. The scheme of an experimental fixed fluidized bed was illustrated in Figure 2. The catalyst used for the experiments was MLC500 of which the properties were listed in Table 4 which also listed the properties of quartz sands. The catalytic decarboxylation rates and the thermal decarboxylation rates were determined for materials B-D, as illustrated in Figures 10–12, respectively. As shown in Figures 10–12, the acid removal rates of materials B-D over solid acid catalyst (MLC500) changed little with the increase of the reaction temperature. The results demonstrated that the activation energy of decarboxylation of petroleum acids over acid catalyst was relatively low. Therefore, the catalytic decarboxylation reaction was easy to occur as long as the petroleum acids could diffuse into the center of the acid catalyst for decarboxylation.

Figure 9. Effect of running time on acid removal rate of petroleum acids: (2) γ-Al2O3; (9) quartz sands. Table 4. Properties of Catalyst of MLC500 and Quartz Sands amount of Lewis acid (µmol · g-1) amount of Bro¨nsted acid (µmol · g-1) particle size/mesh a

N means no detection.

MLC500

quartz sandsa

36.85 23.70 100-400

N N 400

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Figure 10. Effect of reaction temperature on acid removal rate of thermal and catalytic decarboxylation of material B: (red) MLC500, (blue) quartz sands; volume space velocity 9 h-1, catalyst/oil 6.5. Figure 12. Effect of reaction temperature on acid removal rate of thermal and catalytic decarboxylation of material D: (red) MLC500, (blue) quartz sands; volume space velocity 8 h-1, catalyst/oil 7.5.

relatively high molecular mass easily reached over 97% over solid acid catalyst even at reaction temperature of 400 °C. In presence of solid acid catalyst, a modest increase of reaction temperature can easily achieve satisfactory acid removal rates for the petroleum acids with relatively high molecular mass compared with those of relatively low molecular mass. Considering the catalytic cracking reactions by petroleum hydrocarbons, the decarboxylation experiments were conducted at temperature of above 450 °C for high acid crude oils. The acid removal rate of petroleum acids in high acid crude oils over the solid acid catalyst of MLC500 was higher than 97% at 460 °C while only 70% was obtained at 520 °C over quartz sands. Figure 11. Effect of reaction temperature on acid removal rate of thermal and catalytic decarboxylation of material C: (red) MLC500, (blue) quartz sands; volume space velocity 8 h-1, catalyst/oil 7.5.

The acid removal rates of petroleum acids with relatively low molecular mass were able to reach over 97% both by the acid catalyst of MLC500 and by thermal decarboxylation. However, the reaction temperatures of decarboxylation by catalytic method over MLC500 and by thermal method were quite different from one another. For instance, when getting an acid removal rate of 97% of petroleum acids, the reaction temperature for catalytic decarboxylation was low to 300 over MLC500, while that for thermal decarboxylation was higher than 550. One of the possible reasons for the reaction temperature difference between the two methods was their different energy barriers. For example, the energy barrier of catalytic decarboxylation was 125 kJ · mol-1 in the presence of acid catalyst while that of thermal decarboxylation was as high as 293 kJ · mol-1. The acid removal rates of petroleum acids with relatively high molecular mass were difficult to reach with the thermal method. For example, the acid removal rate can only achieve 75% under the reaction temperature of 550 °C for thermal method. However, the acid removal rates of petroleum acids with

Conclusions Developing an economical and effective catalytic process to remove petroleum acids from crude oils is important for the refining industries. The research results indicated that the solid acid catalyst was highly effective for catalytic decarboxylation of petroleum acids from crude oils with an acid removal rate of higher than 97% in a fixed fluidized bed reactor. More attracting was that the solid acid catalysts had a feature of bifunctionality, i.e. a combination of catalytic decarboxylation and catalytic cracking reaction. It is an especially promising practical method to process high acid crude oils. A fluidized catalytic decarboxylation technology combining the catalytic cracking reactions for petroleum hydrocarbons will be a promising technology to process high acid crude oils. Acknowledgment. The authors gratefully acknowledge the financial support of the national basic research program (No. 2006CB202505) of The Ministry of Science and Technology of The People’s Republic of China. They also wish to thank Dr. Cheng Congli for helpful discussions and careful reading the manuscript. EF7006547